One for sorrow, Two for joy

We live next to a bird sanctuary and every evening thousands of black birds make a huge song and dance over which particular branch they want to roost on that night.
I am often in the loft looking out of the sky light when this happens and I have noticed that they get slightly earlier each night, corresponding directly with the earlier sun set.
Clever blighters, but whats even cleverer is that Daisy starts to look out of the window for them just before they start the evenings show.